The may also have been thinking that even a fast sd is slower, and not everyone will use the fastest cards (remember that people who research this stuff are a minority), so they didn't want to get a reputation for making slow phones when it was actually the cards being used this way that was slowing them down. I'm only speculating, but it seems plausible.
Add increased sd wear (and I know from experience that sd cards can suddenly fail) and I personally have no interest in adoptable storage.